For potty purposes, we tend to say peepee a lot here. As in, "Hey! Where's baby's peepee, Mommy?"
In the grown up world I prefer little followed by the person's name. I.E. little michael. I figure it does half the thinking, it deserves half the name.
(It's only little in comparison to the larger whole, btw. :P) I also like wanker.
For testicles, I prefer, family jewels, balls, nads, little friends, etc.